1. Field of Invention
The present invention relates to a water-based electrolyte. More particularly, the present invention relates to a water-based electrolyte for use in an electric double layer capacitor.
2. Description of Related Art
Electric double layer capacitors are also known as supercapacitors. As its name suggests, the supercapacitor stores more energy per unit weight than the conventional capacitor. Furthermore, the electric double layer capacitor exhibits better power rating than conventional rechargeable batteries during electric output. With the advantages such as long shelf life, good cycling characteristics, and wide operating temperature range, the electric double layer capacitors have attracted attentions for their applications in energy storage and output.
Briefly, an electric double layer capacitor includes a pair of polarizable electrodes and a separator interposed therebetween, wherein the electrodes and the separator are immersed in an electrolyte. When a potential difference is applied across the electrodes, cations in the electrolyte move toward the negative electrode whereas the anions in the electrolyte move toward the positive electrode and thereby forms an ionic current within the electric double layer capacitor. When the positively charged- and negatively charged-ions arrive at their respective electrode, the charges would be adhered to the interface between the electrolyte and the respective electrode, whereas the electric field applied on the electrodes may keep the ions thereto and thereby may store the electric energy in the electric double layer capacitor.
Generally, the electrolyte of the electric double layer capacitor may be an electrolyte solution or a molten salt electrolyte. The electrolyte solutions are categorized into water-based electrolytes and organic electrolytes (also referred to as non-water-based electrolyte). Common organic electrolytes consist of tertiary or quaternary ammonium salts dissolved in an organic solvent such as propylene carbonate, acetonitrile or sulfolane. Common water-based electrolytes are aqueous solutions of metal salts, such as IA, IIA metal salts or ammonium salts, dissolved in an aqueous solvent such as KOH or H2SO4 aqueous solution.
Conventional organic electrolytes and water-based electrolytes have their own advantages and disadvantages. For example, organic electrolytes may be charged and discharged at a relatively higher voltage (about 2-3 V), and have a broader working temperature range. However, organic solvent tends to decompose at high working voltage thereby jeopardizing the charging-discharging stability. Also, the organic electrolytes have higher internal resistance, and hence, it is hard to improve the maximum discharging power of the electric double layer capacitor. The organic electrolytes are generally toxic and accordingly require a more stringent packaging condition, which would further increase manufacturing cost. Further, it is hard to control the volatility of the organic solvents and hence the service life of the organic electrolytes is usually short.
In contrast, the water-based electrolytes have the advantages of having a lower internal resistance, and hence a better discharging efficacy. In addition, the water-based electrolytes are usually non-toxic and the manufacturing cost is generally lower than that of the organic electrolytes. Yet, the working voltage of the water-based electrolyte is smaller (generally less than 1 V) due to the breakdown voltage of the solution. Further, to provide better conductivity, the water-based electrolytes are usually strong alkali (pH≧13) or strong acid (pH≦2). However, such solutions are highly corrosive and may seriously damage other elements of the electric double layer capacitor. The corrosiveness of the water-based electrolyte is the main cause of the narrow working temperature range of the electric double layer capacitor. In addition, the operating consistency and packaging stability are also resulted from the corrosiveness of the water-based electrolyte.
In view of the foregoing, there exists a need in the art to provide a novel electrolyte for improving the working efficacy and other operating characteristics of the electric double layer capacitor.